The 3rd generation partnership project (3GPP) has initiated the Long Term Evolution (LTE) program to bring new technology, new network architecture and configuration, and new applications and services to the wireless cellular network to provide improved spectral efficiency, reduced latency, faster user experiences and richer applications and services with less cost. LTE aims at realizing an E-UTRAN (Evolved Universal Terrestrial Radio Access Network).
In a universal mobile telecommunications system (UMTS), when a wireless transmit/receive unit (WTRU) is camped on a cell (i.e., when the WTRU has chosen a cell and is tuned to that cell's control channel), it regularly searches for a better cell according to a set of criteria. If a better cell is found, the better cell is selected and the WTRU will camp on the better cell. In earlier UMTS systems, the WTRU could perform cell reselection either in Idle mode, forward access channel (FACH) mode, or paging channel (PCH) mode. In LTE, there are only two states: LTE_Idle and LTE_Active. The WTRU can perform cell reselection only in the LTE_Idle state.
In previous UMTS systems, before the WTRU decides to camp on a cell, it needs to check some basic criteria for the current cell it is camping on. The conditions Squal >0 (cell selection quality value) and Srxlev >0 (cell selection reception level value) need to be satisfied for a WTRU to camp on a cell, where Squal is measured as:Squal=Ec/Io−Qqualmin  Equation (1)
where Ec/Io (the ratio of the energy of the pilot signal being measured to the total power in the channel) is measured by the WTRU and the value Qqualmin (the minimum required quality level in the cell) is read from system information block (SIB) 3 which is broadcast by the system and includes cell selection and reselection information. Srxlev is measured as:Srxlev=RSCP−Qrxlevmin−max(UE_TXPWR_MAX_RACH−P_MAX,0)   Equation (2)
where the received signal code power (RSCP) is measured by the WTRU and Qrxlevmin (the minimum required reception level in the cell) and UE_TXPWR_MAX_RACH (the maximum transmission power level a WTRU may use when accessing the cell on the random access channel (RACH)) are read from SIB 3. P_MAX is the maximum radio frequency output power of the WTRU.
Other than Qqualmin. Qrxlevmin, and UE_TXPWR_MAX_RACH, a few other parameters are transmitted in SIB 3 and SIB 11 for cell reselection. The following parameters are transmitted in SIB 3.
Sintrasrch (optional) is the threshold for triggering intra-frequency measurements. The WTRU should measure the intra-frequency neighbor cells when Squal≤Sintrasrch. The WTRU will always measure the intra-frequency neighbor cells when Sintrasrch is not specified.
Sintersrch (optional) is the threshold for triggering inter-frequency measurements. The WTRU should measure the inter-frequency neighbor cells when Squal≤Sintersrch. The WTRU will always measure the inter-frequency neighbor cells when Sintersrch is not specified.
SsearchRAT (optional) is the threshold for triggering inter-RAT measurements. The WTRU should measure the inter-RAT neighbor cells when Squal≤SsearchRAT. The WTRU will always measure the inter-RAT neighbor cells when SsearchRAT is not specified.
Qhyst1s is used in ranking the serving cell based on RSCP.
Qhyst2s is used in ranking the serving cell based on Ec/Io.
Qqualmin is the minimum required quality measure in the cell, based on Ec/Io.
Qrxlevmin is the minimum required reception level in the cell, based on RSCP.
UE_TXPWR_MAX_RACH is the maximum allowed uplink (UL) transmission (TX) power a WTRU may use when accessing the cell on the RACH.
Treselection is the value of a timer that indicates an amount of time that the neighbor cell should meet cell reselection criteria for the WTRU to reselect to that cell.
Cell Selection and Reselection Quality Measure are either Ec/Io or RSCP, and specify the measurement quantity that the ranking should be based on.
The following parameters are transmitted in SIB 11, which relate to neighbor cell information.
Neighbor List (work on in LTE to define reduced or no neighbor cell list (NCL)).
Qoffset1s,n is the Quality Offset used to rank a cell based on RSCP.
Qoffset2s,n is the Quality Offset used to rank a cell based on Ec/Io.
UE_TXPWR_MAX_RACH is the maximum allowed UL TX power for a neighbor cell.
Qqualmin is the minimum required quality measure based on Ec/Io.
Qrxlevmin is the minimum required reception level based on RSCP.
Using these parameters, the WTRU is able to rank its serving cell and the neighbor cells. For the UMTS cell, the serving cell ranking is given as:Rank_s=RSCP+Qhyst1+Qoffmbms  Equation (3)
For UMTS cells (neighbor inter-frequency), the neighbor cell ranking is given as:Rank_n=RSCP−Qoffset1+Qoffmbms  Equation (4)
and for GSM cells:Rank_n=RSSI−Qoffset1+Qoffmbms  Equation (5)
Similar ranking equations can be used when the measurement quantity is Ec/Io.
The signaled value Qoffmbms is added to those cells (serving or neighboring) belonging to the multimedia broadcast-multicast service (MBMS) preferred frequency layer (PL).
In LTE since there are two systems to reselect to, UTRAN and GERAN, it also helps to look at the GERAN-UTRAN cell reselection procedure. The following are three parameters used in GERAN to UMTS cell reselection.
Qsearch_I: if the strength of the serving GSM cell falls below this value, then the WTRU will start taking measurements.
FDD_Qoffset: the amount that the WCDMA cell's RSSI must be higher than the serving GSM cell's RSSI.
FDD_Qmin: a minimum threshold for Ec/No for UTRAN FDD cell re-selection.
The radio resource layer (RR) in GSM compares EcIo_Avg with FDD_Qmin_dB. If EcIo_Avg is less than FDD_Qmin_dB, then there is no cell reselection. If EcIo_Avg is greater than FDD_Qmin, then compare the WCDMA cell to the serving GSM cell. The WCDMA neighbor cell is better if the WCDMA cell's RSCP is greater than the GSM cell's RSSI+FDD_Qoffset for FDD_Qoffset≠0 or if the WCDMA cell's RSCP is greater than the GSM cell's RSSI for FDD_Qoffset≠0.
Using the above criteria for cell reselection, a number of factors like cell loading and WTRU bandwidth capability have not been considered. These factors are relevant in LTE with orthogonal frequency division multiplexing (OFDM) as the physical layer medium. In addition, there are other factors or drivers that may be considered in cell reselection which are described in the table below. In the table, L→L indicates LTE to LTE inter-frequency mobility; L→U indicates LTE to UTRAN inter-RAT mobility; U→L indicates UTRAN to LTE inter-RAT mobility; L→G indicates LTE to GERAN inter-RAT mobility; and G→L indicates GERAN to LTE inter-RAT mobility. In the table, a value of “X” indicates that the driver is required, a value of “(X)” indicates that the driver is optional, and a blank space indicates that the driver is not required.
TABLE 1Drivers for Mobility Control during IDLE (cell reselection) forInter-RATApplicabilityL →L →U →L →G →Necessary features to support#DriversLULGLdrivers1RadioXXXXXInter-frequency/RATconditionmeasurements (solutions tomitigate measurement loadshould be considered, e.g., S-criteria);Cell reselection and reselectioncriteria.2Camp loadXXX(X)(X)Mechanism to prioritize cellbalancingreselection to certain layer/RAT,depending on the loading oflayers/RATs;Load information exchange (notneeded if balancing isinadaptive, i.e., only based onsubscriber penetration on eachband/RAT).3Traffic loadN/Abalancing4WTRU(X)XXXXMechanism to prioritize cellcapabilityreselection to certain layer/RAT,depending on the WTRUcapability.5HCS(X)(X)(X)(X)(X)Mobility detection (e.g., numberof crossed cells);Mechanism to prioritize cellreselection to certain layer/RAT,depending on the WTRU speed(e.g., HCS mechanism as inUTRAN).6NetworkXXXX(X)Mechanism to direct the WTRUsharingto the appropriate PLMN at anetwork sharing border;Mechanism to restrict WTRUmeasurements and reselection tocells that are entitled to access.7PrivateX(X)X(X)Mechanism to prioritizenetworks/homereselection to private/home cellscellsthat are entitled to access;Mechanism to restrict WTRUmeasurements and reselection tocells that are entitled to access;Other unidentified features,FFS.8Subscrip-XXX(X)(X)Mechanism to prioritize celltion/Policyreselection to certain layer/RAT,baseddepending on the subscriptionmobilityinformation or any othercontroloperator policy (e.g., for L → Lthere may be cases where anoperator has policy in allocatingWTRUs to certain frequenciesdue to different carrierbandwidths).9ServiceN/Abasedmobilitycontrol10MBMSX(X)XMechanism to prioritize cellreselection to the layer/RAT,depending on whether theWTRU requires reception of acertain MBMS transmission.
A procedure needs to be defined on how the WTRU and the network would signal these parameters to each other and how the WTRU would use these parameters in its cell reselection equations. Moreover, some parameters for cell reselection can hold priority over other parameters in some scenarios. These factors also have to be taken into account in designing an overall procedure for cell reselection. Since there are two systems to which LTE could reselect to, it also needs to be seen whether some parameters could be optimized for cell reselection.
Some parameters for inter-RAT cell reselection have been proposed and are mentioned below. The present disclosure describes the inter-RAT cell reselection algorithm, mentioning the offsets and radio condition-based parameters that need to be signaled for inter-RAT cell reselection from LTE. Also proposed is how the cell reselection algorithm could be defined in LTE to take into account both UTRAN and GSM systems.